Power line carrier frequency telephone system



June 13, J. M PCWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM 2 Sheets-Sheet 1 Filed Oct. 1l, 1946 /NVENTOR JM. BA RSTOW A TTORNEV June 13, 1950 J. M. BARsTow POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM 2 Sheets-Sheet 2 Filed Oct. l1, 1946 N ...um

4 Ils/VENTO@ v 5y JM. BARSTOW Patented June 13, 1950 POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM John M. Barstow, Mountain Lakes, N. J., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 11, 1946, Serial No. 702,735

6 Claims. (Cl. 179-25) This invention relates to communication systems and particularly to telephone systems including subscriber stations arranged for carrier current operation over a power transmission line.

Objects of the invention are the provision of carrier telephone service over power transmission lines, the provision of means for automatically guarding the telephone vline vwhich connects the power line to a telephone central cnice on calls between telephone stations connected to the power line, the elimination of manual control means heretofore provided at such stations for use only on such calls, and in general the simplication and improvement of power line carrier telephone systems.

This invention is a telephone system including subscriber stations arranged for carrier current operation over a power transmission line, carrier current of one frequency being used for transmitting over ,thel power line `voice and signal currents to any one of these stations and carrier current of another frequency being used vfor transmitting over the power line voice and signal currents from any one of these stations. On reverting calls the carrier frequencies for transmitting voice and signal currents to and from calling station are automatically interchanged,

A feature of the invention is a carrier terminal station or equipment unit connecting a telephone line from the telephone central oice to a power transmission line, the carrier terminal equipment comprising a voice amplifier and relay responsive to voice frequency currents incoming over the telephone line from the central office, the operation of this relay being eective to energize the carrier transmitter and thus effect the transmission of carrier current from the carrier terminal over the power line. Because on a call between stations connected to the power line, no voice frequency current is transmitted from the telephone central oiice after the called station answers the call, the voice frequency relay in the terminal equipment remains normal and no carrier current is transmitted from the carrier terminal equipment while conversation takes place between the calling and called stations. r Understanding of the invention will be facilitated by considering a system in which the invention and its features are embodied, such a system being represented schematically in the drawings. The invention is, however, not limited in its application to the particular system shown but is generally applicable to telephone systems.

The drawing, which consists of two figures, represents an automatic telephone exchange serving subscriber stations some of which are arranged for carrier current operation over a power transmission line.

Fig. 1 shows an automatic telephone exchange comprising a subscriber station I0, subscriber lines Il and 2| and line circuits I2 and 22, line finder switches LFI and LF2, selector switches Sl and S2, connector switches CI and C2, carrier terminal equipment CT individual to the line 2 l, and a power transmission line PL to which the carrier terminal equipment CT is coupled by coupling means M.

Fig. 2 shows one of a plurality of subscriber stations connected to, and arranged for carrier current operation over, power line PL.

`The subscriber station Il] is of the usual type provided for use on lines terminating in an automatic central oce and includes a dial or other impulse sender (not shown) for use in selectively controlling the switches through which desired connections are established. The line finder, selector and connector switches are of the two-motion step-by-step type, each of the line finder and selector switches being represented in thev drawing by a set of brushes and a single set of terminals and each of the connector switches being represented by a set of brushes and two sets of terminals. The relays and other apparatus associated with the line finder, selector and connector switches are represented by broken lines. Reference may be had to Automatic Telephony, by Smith and Campbell, 2nd edition, pages 53 to 65, inclusive for a detailed description of the structure of such switches and their operation when used as selectors :and connectors. Reference may be had to the patent to R. L. Stokely, No, 1,799,654, granted April 7, 1931, for a detailed disclosure of a line circuit and a line nder of the step-by-step type. The ringing code generator 21 comprises a source of ringing current and means for interrupting the ringing supply conductors as required to produce a plurality of different ringing codes for signaling subscriber stations. Reference may be had to the patent to Duguid, No. 2,225,907, granted December 24, 1940, for disclosure of such a ringing code generator and to the patent to Kcechling, No. 2,289,503, granted July 14, 1942, for a detailed disclosure of a connector switch arrangedl for ten-party code ringing.

The carrier terminal equipment CT, power line i PL, and the carrier telephone stations of Fig. 2

constitute a power line carrier telephone system similar generally to the system disclosed in the copending application of R. C. Edson and J. W. Emling, Serial No, 653,254, led March 9, 1.946, now

prises a hybrid coil or group of transformerwindings 30 arranged to transmit signal and voice currents to and from line 2|. The resistor 3| and condenser 32 constitute a network connected to the middle right pair of windings y to balance the line 2|. The upper pair of windings receive voice frequency currents from a carrier receiver comprising the amplifier-demodulator 50; and the lowest pair of windings transmit voice frequency currents from line 2| to the input circuit of a carrier transmitter comprising oscillator-modulator-amplier 40. The power line PL is connected through coupling means 44 and conductors 43 to the carrier receiver comprising receiving band filter 69 and amplier-demodulator 50, and to the carrier transmitter comprising transmitting band filter 4| and oscillator-modulator-amplier 30. The amplier-demodulator 50 and oscillator-modulator-amplifier 40 may be of any known suitable type. The oscillator-modulator-amplier150 1s tuned to generate and transmit through filter 4|, carrier current of a particular frequency, designated Fl, modulated by either signaling or voice currents incoming to hybrid coil 3a from line 2|. The amplifier-demodulator 50 receives, through filter 49 from the power line PL, carrier current vof another particular frequency, designated F2, modulated by voice and signal currents from any one of the carrier telephone stations connected to the power line PL. The transmitting and receiving filter 4| and i9 transmit the voice frequency side bands in addition to the carrier frequency. The carrier terminal equipment CT further comprises relay 35 controlled by ringing current incoming over line 2l to energize the oscillator-modulator-amplifier I) and thereby transmit impulses of carrier current over the power line correspondingto the ringing code received by relay 35 from line 2|. The carrier terminal equipment CT further comprisesl relays 5| and. 52 controlled by carrier current of frequency F2 received over power line PL by amplifier-deniodulator 50, an amplifier and detector '60 responsive to voice frequency current incoming over line 2|, a relay 6| controlling the energization of oscillator-modulatoramplier 40, and an auxiliary ringing relay'38 which controls the locking winding of relay 6|. A resistorZl, connected in series with the middle left windings of hybrid coil 30 and a resistor 2B and condenser '29, knormally connected in parallel with these windings constitute a network which in combination with line 2| have an impedance equal to that of the balancing network 3|, 32; whereby a good balance is obtained on reverting calls so that relay 6| will not operate responsive to voicek modulated carrier current of frequency F2 incoming over the power line from the called station as hereinafter described. The functions of each of the relays is hereinafter described in detail.

Each of the carrier telephone stations is similar to the one shown in Fig. 2 and is connected through individual coupling means 69 to the power line PL. Each station comprises a carrier receiver, a carrier transmitter, a telephone receiver 81| and transmitter 85 combined in a handset which normally ,rests on the receiver hook 83, a dial 86 for controlling automatic switches in the central office to establish desired connections, a ringer '|9 and a ringing relay vit connected to demodulator '|3 for actuating lthe ringer '19 responsive to ringing modulated carrier current of frequency FI. The carrier receiver comprises a receiving band filter tuned to the carrier frequency FI, a receiving band filter I2 -tunedto the carrier frequency F2, and amplifier-demodulator apparatus 13. The carrier transmitter comprises loscillator-modulator-ampliiier apparatus 88, a transmitting band filter 89, a frequency control relay 90, oscillator crystals 9| and 92, and a relay 93 for connecting the output conductors of filter 89 to the power line. Each of the receiving and transmitting lters transmit the voicefrequency'side bands in addition to the carrier frequency.

The oscillator-modulator-amplier apparatus, conventionally illustrated as a rectangle 88, includes an oscillator whose frequency is controlled by the one or the other of the crystals 9|, 'S2 depending upon the operated or unoperated condition of relay '90, amplifying and modulating means whereby the carrier waves are modulated at times in accordance with voice waves applied through transformer '81. The oscillator may be any suitable and well-known crystal controlled oscillator such as found shown in Cadys book on Piezoelectricity, first edition, 1946, by Mc- Graw-'Hill vBook Company, Incorporated, pages 495 and 499, Taylor Patent 1,639,817, dated August 23, 1927, or Nicholson Patent 2,212,845, dated August 27, 1940. The one or the other of the two crystals '9| or 92 is switched into circuit to establish the .carrier frequency at one value or another at different times as herein described. Modulation may be accomplished in any suitable well-known manner by means of any one of many modulating circuits 'of which the following examples are given, Van der Bijl Patent 1,350,752, August 24, 19.20, and I-Iesing Patents 1,343,562, June 15, 1920, and 1,560 ,054,November 1'1, 1925. Lack Patent 2,218,200, October 10, 1940, discloses a suitable-combined oscillator and modulator which could readily be arranged for shift from one crystal to another by relay 90. Devices of this kind are considered to be so well known in the art that they need no further description.

The power supply'for the oscillator-modulatoramplier and dernodulator apparatus at each carrier subscriber station includes a rectier 61 and a -transformer 66 connectedr to the power line VPL either directly or through an intermediate transformer. Other transformer windings may .supply the energy for heating the filaments of vacuum tubes and supply whatever other operating potentials are required.

Call from station 10 to a carrier telephone station Assume that a call is initiated at station l0, that the line Il is connected by a line finder LF! to a selector Si, that the selector SI is selectively controlled by dial impulses from station I0 to select a group of trunks and an idle trunk thereinrassociated with a connector CI, and that the connector Cl is selectively controlled by dial impulses from station lt to select 'stations connected to the power line.

the line 2|. n Assume further that the line 2| ls idle when selected and tested by connector Cl,

that a ringing code is Selected responsive to dial impulses from station I8, and that the selected ringing code is transmitted through the brushes Vthe input conductors of modulator-amplifier 4U,

whereby ringing modulated carrier current of frequency FI is transmitted through filter 4|, conductors 43, and coupling means 44, over the power line PL, to each of the carrier telephone Relay 38 locks independent of relay 35, opens the locking path of relay 6 I, connects ground to the cathode of gas-filled tube 55 and connects the positive pole of battery B2 through resistors 31 and 54 to the control anode of tube 55. The tube 55 is not energized at this time since the control gap is short-circuited by ground at a front contact of relay 35 and also at a back contact of relay At each of the telephone stations connected to line PL, the incoming ringing modulated carrier current of frequency Fl is transmitted through a coupler 69, conductors 15, and a normally closed contact of the springs controlled by receiver hook 83, through filter 1| to the input circuit of amplier-demodulator 13. The output circuit of amplier-demodulator 13 includes resistor 14 and the winding of relay '15 and relay 16 is operated responsive to carrier current incoming over the power line. A condenser 15, connected in parallel with the wind- "ing of relay 16, renders this relay somewhat slow in operating so as to tend to prevent its operation responsive to interference currents such as static. The output conductor of demodulator 13 is further connected through condenser 6| and the olf-normal contact of dial 86 to telephone receiver 84, in parallel with resistor 82. While the telephone handset is in normal position on hook S3, the receiver 84 and resistor 82 are shortcircuited at a back contact of the receiver-hook contact springs. The operation of relay 16 closes a vcircuit from the left lower winding of transformer 66 through rectier 61, conductor 68, re-

' sistor 8D, front contact of relay 15, condenser 11,#

and through the windings of ringer 19 to ground at a back contact of the springs of hook 83. The

windings of ringer 19 are thereby energized while 'condenser 11 is charging. Since the incoming 'carrier current of frequency FI is modulated by the ringing current over line 2|, relay 16 is alternately operated and released at the ringing 'I current frequency (for instance 16 cycles per second) during each ringing interval of the cordance with the ringing code of the particular station which is being called.

When the handset is removed at the called station, the circuit through ringer 19 is opened, the short circuit across the receiver 84 is opened, and the positive potential supply conductor 68' jresistors 56 and 31.

ls connected to the winding of relay 93 and to the oscillator-modulator-amplier 88. Relay 93 is somewhat slow in operating to allow time for relay to operate if relay 16 is operated at the time the handset is removed, as is the case at the calling station on a reverting call as hereinafter described. However,l on the call being described, relay 16 is not operated at the called station when the handset is removed because at this time there is no carrier current being transmitted over the power line; and therefore relays 16 and 80 are not operated. If the receiver were lifted during ringing while relay 16 was operated, relay 96 would operate to connect iilter 12 into' the receiving circuit, but there being no carrier power of frequency F2 on the power line, and relay 93 being slow to operate, relays 16 and 9|] would immediately release. Relay 63 connects the output conductors of filter 89 across conductors 10, and'connects the input conductors of lter 1I, through a receiver hook contact and a back contact of relay 95, across conductors 1).

`Relay 16 is thereupon operated responsive to normal, the oscillator crystal 92 is connected,

through the innerleft back contact of relay Sil, in the oscillator cir-cuit oi oscillator-modulatoramplifier apparatus 88; and carrier current of frequency F2 is transmitted from the amplifier of apparatus 88, through lter 89 and a iront contact of relay 93, through conductors 15 and coupling means 69, over power line PL to each of the other carrier stations and to the carrier terminal equipment CT. At the other carrier telephone stations, the carrier current of frequency F2 does not reach the demodulator 13 because the input circuit of filter 12 is open as long as the handset is in normal position on hook 83; but, at the carrier terminal equipment CT, the carrier current of frequency F2 passes through coupler 44, conductors 43, and receiving band lter 49 to the input circuit of amplier-demodulator 56, causing the operation of relay 5|. The operation of relay 5| closes a loir-cuit for operating slow-to-release relay 52 and closes a, direct current path through the middle left pair of windings of hybrid coil 3G across the conductors of line 2|, thereby to op- Aerate the ringing trip relay (not shown) of the lines I| and 2|. The operation of relay 52 opens the circuit iconnection between the winding of relay 35 and line 2|. With relay 35 released and relay 5| operated, the aforementioned short circut across the control gap of tube 55 is opened and condenser 51 starts to charge in series with After a predetermined interval of time, for instance, .6 second during which condenser 51 is charging, the voltage across the control gap of tube 55 will reach the discharge value whereupon the right winding of relay 3B is energized in series with the main gap of tube 55. Since the windings of relay 38 are diierentially energized, relay 38 releases and the tube 55 is extinguished. The release of relay 38 prepares the locking circuit of relay 6|; so that after relay 6| is operated, it will maintain the energization of amplienmodulator 40 during fconversation between th calling station .ID 'and the called station `connected to vthe power line.

'The `delay in :the Abreakdown 'of the :tube 55 :and

release vof relay 33 .allows relay -6| to release operated due to surges Aresulting from tripping oif the ringing current. Voice currents incoming over line 2| from station ||l are transmitted through the lowest pair of windings yof hybrid coil 38 to the input conductors of voice amplier and detector 60 to effect the operation of relay 5|. Relay 5| locks under the control 'of relays 38 and 5|, connects battery B2 Vto oscillator-modulator-amplifier '48 zand thereby effects the transmission of carrier current of frequency Fl over the power line PL, opens the :bridge through resistor 28 and condenser 29 and shortcircuits resistor 21. The voice currents from line 2l modulate the carrier current of .frequency FJ transmitted from oscillator-modulator-amplier 40, through filter 4| over the power line 'PL to each of the carrier telephone stations. At `the called carrier telephone station, the carrier current of frequency FI incoming through filter 1I to amplier-demodulator 13 effects the operation of relay 16 and the demodulated voice currents are transmitted through receiver 84. At each of the other carrier telephone stations, relay 1-.6 is operated responsive to the carrier current of frequency FI incoming to the amplierdemodu lator 13, but the receiver 84 is short-circuited; and, since relay 16 is continuously energized, no operation of the ringer 19 results. At the called station, voice currents from the transmitter 85 are transmitted through coil 81 to the input circuit of modulator 88, therein to vmodulate the carrier current of frequency F2 which is being transmitted over the power line PL, through coupler All, conductors 43 and filter 49 tothe .ampliiier-dernodulator 50 of carrier terminalequipment CT, the voice currents being further transmitted through hybrid coil .3.0 over line 2| to the calling station I0. Thus talking proceeds between the calling station l0 and the Lcalled carrier telephone station.

When the handset is replaced on hook 83 at the Vcalled station, the receiver 84 is shortv deenergize the oscillator-modulatcr-amplifier 4,0

and thus terminate the transmission of carrier current of frequency FI over the power line. When the transmission of carrier current vof frequency FI over line PL ceases, relay 16 releases at each of the carrier subscriberstations. When the receiver is restored to normal at the calling station Il), the connector Cl, selector-Sl and line iinder LFI are restored to normal in usual and well-known manner.

Call from carrier telephone station Assume next that the handsetis removed `from hook 83 at one of the carrier stationsconnected to power line PL to initiate a call. .Relay 93 is thereby operated, relay 90 remains normal, Yand -l the oscillator-modulator-amplier 88 is energized :to transmit *carrier current -of yfrequency F2 over .power line PL, causing the successive operation 'of relays 5| and 52 `of the carrier terminal equipment CT, as above described. vAt each of the other carrier stations, the carrier current of frequency P2 transmitted over line PL from the calling station is ineffective to operate relay 16 because the input conductors 10 are normally lconnected across the input of -lter 1| and are not normally lconnected across the input conductors of filter 12. The operation of relay 5| Icloses the bridge across Iline 2| to operate the line relay (not shown) of line circuit -22 and thus start the operation of an idle line finder LF2 to connect AWith line 2|; and line 2| is thereby connected to the associated selector S2. The operation of relay 52 opens the circuit connection between relay `35 and -line 2|. Dial tone is transmitted from a selector S2 over line 2|, through hybrid coil 30, to the input conductors of modulator 40 and to the input conductors of voice amplier and detector 6D, causing the operation of relay 6|. Relay 6| locks vand connects battery yB2 to oscillator-rnodulator-amplier 4 0 to effect the transmission Vof dial tone modulated carrier current of frequency -FI through filter 4| over power line PL to the carrier telephone stations. The carrier current of frequency F| eiects the operation of relay 1.8 at each of these stations; and, at the calling station, .the dial tone is transmitted through receiver ;84 to indicate that .dialing of the number of the telephone station with which connection is desired, may be started. Each time a digit is dialed, the receiver is disconnected by actuation of the oif-normalcontacts of the dial from the ampliier-dernodulator 13; and the impulse contacts .ofthe dial alternately remove and apply energizing power to the oscillator-modulator-amplifier 88 to modulate the carrier current of frequency F2 being transmitted over line PL. At the carrier terminal equipment CT, relay 5| is alternately vreleased and reoperated responsive .to each dial impulse of the train of impulses transmitted over line PL corresponding to a dialed digit; and relay 5| thus repeats each train of dial impulses Vover yline y2| to control the selector S2 .and a connector C2 to `establish connection with the called line and to select the ringingcodewhich is to be .transmitted over thecalled -line to .signal the called subscriber station. Relay 52 is .sloW-.to-release and remains operated during the response o'f relay 5| to dial impulses. Relay 6| may or may not release during dialing. 9

Assume rst that the called station is not another one ofthe carrier subscriber stations connected to power .line PL. `Ringing current of ythe selected code .is vtransmitted through the brushes of connector C2 over the .called line and, when 'the called subscriber answers the call, the ringing circuitis opened andthe talking path is completed 'through connector .C2 between the calling and called stations. If .relay .6| .released during dialing, itis reoperated when 'the called party answers. ARelease of .the yconnection is .under the control of the calling station. When the handset is restored to .normal at the calling carrier subscriber station, relay I9'3re1eases and oscillatormodulator-amplifier $8 .is deenergized, terminating the transmissionof vcarrier current of frequency F2 over power .line PL and thus causing the successive Vrelease ofrelays 5| and 52 of carrier terminal equipmentCT. T-he release of relay 5| opens the locking .circuit of .relay 6| and opens the bridge across line 2|` 11o/effect the return o( nected to power line PL, such a call being known as a reverting call. In such a case, the connector C2 finds the called line 2| busy when it makes the usual busy test; and a busy tone is trans- '.mitted from connector C2 through selector S2 and line finder LF2, over line 2 I, through hybrid -coil 3U to modulator-amplifier 40 and to voice amplier and detector 65. Relay 6| operates, connecting battery B2 to amplifier-modulator 40; and busy tone modulated carrier current of frequency Fi is transmitted through filter 4|, power line PL, filter 1|, to amplifier-demodulator 13, and the busy tone is further transmitted through condenser 8| to the telephone receiver 84 ,at the calling station. The connector C2 is arranged to make a reverting call test to determine whether the called line is busy because the call is a reverting call. If the call is a reverting call the line finder LFZ and selector S2 are restored to normal when the calling subscriber replaces the receiver; but the connector C2 remains connected to line 2| to ring the called station. Reference may be had to the patent to H. Hovland, Patent No. 1,849,087, granted March l5, 1932, for disclosure of a connector arranged to make the aforementioned reverting call test. Upon hearing the busy tone transmitted from connector C2, the calling subscriber restores the handset to normal on the receiver hook 83. When the handset is hung on the hook 83, relay 93 releases and the oscillator-modulatoramplifier apparatus 88 is deenergized thereby terminating the transmission of carrier current of frequency F2, thereby causing the successive release of relays and 52 of the carrier terminal equipment CT. The release of relay 5| opens the locking circuit of relay 5| and opens the loop across line 2| to effect the aforementioned release of the line finder LF2 and selector S2 and to effect the closing of the ringing circuit in connector C2, whereby ringing circuit is transmitted over line 2| to operate relay 35 according to the ringing code of the called station. The release of relay 52 closes the bridge consisting of condenser 33 and rectifier 34 across line 2|, so that relay 35 is operated by the ringing current. Relay 35 connects line 2| through ccndensers 36 to the input conductors cf modulator-amplifier 45 and connects battery B2 to oscillator-modulator-arnplier 40 so that ringing current modulated carrier current of frequency Fl is transmitted through filter 4| over line PL to each of the carrier subscriber stations to actuate the ringers according to the ringing code of the called station. The operation of relay 35 also causes the operation of relay 38 and relay 38 locks independent of relay 35.

When the handset is removed from hook 83 at the called station to answer the call, relay 93 is L operated, oscillator-modulator-amplier 88 is energized to transmit carrier current of frequency F2, and the input of filter 1| is connected across conductors Til. The carrier current of frequency F2 transmitted over line PL from the answering path through connector C2. With relay 35 released and relay 5| operated, the short circuit across the control gap of tube 55 is opened; and .6 second later tube 55 is energized andthe right winding of relay 38 is energized to cause the release of relay 38 as hereinbefore described. With relay 5| operated and relay 38 released, relay 5| would lock if operated; but on a reverting call no voice frequency current is received over line 2| after relay 38 releases so that relay 6| is not operated. With relay 6| normal, the oscillatormodulator-amplifier 4|] remains deenergized after ringing is tripped in the connector C2 sothat carrier current of frequency FI is not transmitted over the power line PL from the carrier terminal equipment CT. Since relay 6| is normal, the network comprising resistors 21 and 28 and condenser 29 is connected between line 2| and hybrid coil 3|] to balance network 3|, 32 and thereby prevent operation of relay 6| by voice modulated carrier current transmitted over line PL.

When the operation of the ringer at the calling station ceases due to answer at the called station, the calling subscriber removes the handset from hook 83, thereby closing the connection from power line PL through conductors l!) and filter T2 to the input conductors of `amplifierdemcdulator 13. At this time carrier current of frequency F2 is being received over line PL from the calledstation, and the current in the output circuit of amplifier-demodulator 13 operates relay 15; and before the slow-to-operate relay 93 responds to the removal of the handsetv from hook 83, relay 16 closes a circuit for operating relay 9D. Relay 95 locks independent of relay 93 and includes crystal 9| in place of crystal 92 in the oscillator circuit of oscillator-modulator 93 at the calling station to transmit carrier current of frequency FI over line PL to the called station. When relay 93 operates, it connects the output of filter 89 across conductors 1G. With relay 9|] operated at the calling station, the connection between conductors 10 and the input of filter 12 .is maintained until the handset is restored to normal. The calling and called stations are now in condition for conversation over the power line, voice currents from the transmitter at the calling station being effective to modulate the carrier current of frequency FI transmitted from the oscillator-modulator-arnplier 88 at the calling station, and voice currents from the transmitter 85 at the called station being effective to modulate the carrier current of frequency F2 transmitted from the oscillator-modulatoramplier 88 at'the called station.

The release operations on a reverting cal1 vary according to the order in which the calling and called subscribers disconnect. If the calling subscriber hangs up first, relays 90 and 93 release and oscillator-modulator-amplifier 88 is deenergized at the calling station; but no change occurs at the carrier terminal equipment CT until the called subscriber also hangs up. When the called subscriber hangs up, relay 93 releases and the oscillator-modulator-amplier 88 is deenergized thus terminating the transmission of carrier current of frequency F2 over power line PL and relays 5| and 52 of the carrier terminal equipment CT release in succession. The release of relay 5| opens the bridge across line 2| to effect release of the connector CT.

If the called subscriber hangs up first, relay 93 releases and oscillator-modulator-amplier 88 ,iS deenergized tov terminate the transmission `of carrier current of frequency F2 from the called station and thus cause the release of relays 5I and 52 of carrier terminal equipment CT and the release of relay 'I5 and 90 at the calling station. The release of relay 9U at the calling station connects crystal 92 into the oscillator circuit of oscillator-modulator 83 in place of crystal 9|, whereby the carrier current transmitted therefrom is current of' frequency F2 instead of frequency' FI. Thus relay 5I is reoperated soon enough to prevent the release of connector C2 until the handset is hung up at the calling station; at which time, relay 93 releases and oscillator-modulator-amplier 88 is deenergized at the calling station, relays 5| and 52 are released at the carrier terminal equipment CT, and connector C2 is restored to normal.

In case the called subscriber fails to answer, the calling subscriber answers and carrier current of frequency F2 is transmitted over line PL to operate relays 5I and 52 and release relay 58 of the carrier terminal equipment CT, Ringing is tripped due to the operation of relay 5I; and, when the calling subscriber hangs up, relays 5I and 52 release at the carrier terminal, and relay 93 releases and oscillator-modulator-amplifier 88 is deenergized at the calling station.

What is claimed is:

1. In a telephone system, a telephone line, a power transmission line, a carrier terminal station interconnecting said lines, a plurality of telephone subscriber stations connected to said power line, each of said subscriber stations arranged to transmit carrier current of either'one of two frequencies and receive carrier current of the other of said frequencies, said terminal station comprising a normally deenergized carrier current transmitter for transmitting carrier current of one of said frequencies modulated by voice and signal currents incoming over said telephone line and a carrier current receiver for receiving voice frequency modulated carrier current of the other of said frequencies over said power line, a relay at said carrier terminal station controlling the energization of the carrier current transmitter thereat, and means responsive to Voice currents incoming over said telephone line to said carrier terminal station for energizing said relay.

2. In a telephone system, a telephone line, a power transmission line, a carrier terminal station interconnecting said lines, a plurality of telephone subscriber stations connected to said power line, each of said subscriber stations arranged to transmit carrier current of either one of two frequencies and receive carrier current of the other of said frequencies, said terminal station comprising a normally cleenergized carrier current transmitter for transmitting carrier current of one of said frequencies modulated by voice and signal currents incoming over said telephone line and a carrier current receiver for receiving voice frequency modulated carrier current of the other of said frequencies over said power line, and control means at said carrier terminal station for energizing the carrier transmitter thereat, said control means comprising a voice frequency amplifier and detector connected to receive voice currents incoming over said telephone line to said terminal station and a relay operatively energized in response to voice current incoming to said amplifier and detector.

3. In a telephone system, a telephone line, a power transmission line, a carrier terminal station interconnecting said lines, and a plurality of telephone subscriber stations connected to said power line, each of said subscriber stations comprising carrier current transmitting and receiving means arranged to transmit carrier current of either one of two frequencies and to receive carrier current of the other of said frequencies, said carrier terminal station comprising a normally deenergized carrier current transmitter for transmitting carrier current of a first one of said frequenciesv over said power line, means for modulating the carrier current transmitted over the power line with voice currents incoming over said telephone line, a relay controlling the energization of said transmitter, and voice amplifier and detector means connected to said telephone line to operate said relay in response to voice currents incoming over said telephone line to said carrier terminal station.

4. In a telephone system, a telephone line, a power transmission line, a carrier terminal station interconnecting said lines, and a plurality of telephone subscriber stations connected to said power line, each of said subscriber stations comprising carrier current transmitting and receiving means arranged to transmit carrier current of either one of two frequencies and to receive carrier current of the other of said frequencies, said carrier terminal station comprising a normally deenergized carrier current transmitter for transmitting carrier current of a first one of said frequencies over said power line, a carrier current receiver for receiving over said power line carrier current of the other of said frequencies, a hybrid coil having windings connected in series with said telephone line and windings connected tothe input conductors of said carrier transmitter, means including said windings of the hybrid coil for modulating with voice current received over said telephone line the carrier current o1 said first frequency transmitted over said power line, amplifier and detector means connected to said hybrid coil in parallel with said carrier transmitter, and means controlled by said amplier and detector means for energizing said carrier transmitter in response to voice currents incoming over said telephone line.

5. In a telephone system, a telephone line, a power transmission line, a carrier terminal station interconnecting said lines, and a plurality of telephone subscriber stations connected to said power line, each of said subscriber stations comprising carrier current transmitting and receiving means arranged to transmit carrier current of either one of two frequencies and to receive carrier current of the other of said frequencies, said carrier terminal station comprising a normally deenergized carrier current transmitter for transmitting carrier current of a first one of said frequencies over said power line, carrier current receiving means tuned to receive carrier current of the second one of said frequencies incoming over said power line from any one of said carrier subscriber stations, means for modulating the carrier current transmitted over the power line. with voice currents incoming over said telephone line, a relay controlling the energization of said transmitter, voice amplifier and detector means connected to said telephone line to operate said relay in response to voice currents incoming over said telephone line to said carrier terminal station, and means including a locking winding of said relay for maintaining the energization of said transmitter independent of said amplifier and detector means responsive to carrier current of said second frequency in- 13 coming to said carrier current receiving means over the power line from any one of said subscriber stations.

6. In a telephone system, a telephone line, a power transmission line, a, carrier terminal station interconnecting said lines, and a, plurality of telephone subscriber stations connected to said power line, each of said subscriber stations comprising carrier current transmitting and receiving means arranged to transmit carrier current of either one of two frequencies and to receive carrier current of the other of said frequencies, said carrier terminal station comprising a normally deenergized carrier current transmitter for transmitting carrier current of a iirst one of said frequencies over said power line, a carrier current receiver for receiving over said power line carrier current of the other of said frequencies, a hybrid coil having windings connected in series with said telephone line and windings connected to the input conductors of said carrier transmitter, means including said windings of the hybrid coil for modulating with Voice current received over said telephone line the carrier current of said first frequency transmitted over said power line, amplifier and detector means connected to said 14 hybrid coil in parallel with said carrier transmitter, means controlled by said ampliiier and detector means for energizing said carrier transmitter in response to voice currents incoming over said telephone line, and means responsive to carrier current of said other frequency incoming to said carrier current receiver over said power linev from any one of said carrier subscriber stations for maintaining the energization of said carrier transmitter independent of said amplifier and detector means.

JOHN M. BARSTOW.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,773,613 Clark Aug. 19, 1930 1,958,166 Laurent May 8, 1934 2,200,559 Mitchell May 14, 1940 2,289,048 Sandalls, Jr. July 7, 1942 2,301,223 Mitchell Nov. 10, 1942 2,388,001 Loughren Oct. 30, 1945 2,430,471 Lang Nov. 11, 1947 

